The oxidizer shouldn't affect your mileage much, it's some of the other stuff that California required that can but it sounds like you've taken care of most of it. I have an '85 Calif. wagon that gets 25-26 highway and 21-22 city (if my wife drives) and 18-20 city (if I drive). If I slow down I can get better mileage but I do like making that turbo earn its keep. Lugging all that steel up and down hills in San Francisco definitely takes extra fuel (and brakes). Even so the Calif version does loose a 1-2 mpg compared to the 49 state cars.

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LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold